Semiconductor optical amplifier devices (hereinafter referred to as “SOA devices”) are devices comprising an optical amplifying medium typically in the form of at least one striped optical waveguide, that is, “an active stripe structure”. When used as amplifiers, SOA devices are amplifiers for amplifying optical signals without converting it into an electric signal. The structure of these devices is basically the same as semiconductor lasers. In contrast to laser devices, the SOA devices typically have connecting end portions covered with anti-reflecting coatings to reduce their light reflectance, so that SOA devices are typically not of an optical resonator structure. Therefore, a SOA device typically outputs amplified light due to stimulated emission in response to input optical signals, but suppresses laser oscillation.